Conventional refrigeration systems achieve cooling by allowing a refrigerant such as ammonia or a fluorocarbon to evaporate in the coils of an evaporator. As the refrigerant evaporates, it absorbs heat from the surrounding area. A fan or other air moving device is used to draw air through the evaporator so that heat is removed more effectively from the air in the space that is being refrigerated.
As the temperature in the evaporator is generally below the freezing point of water, water vapor in the air often condenses on the evaporator coils and solidifies as frost. The buildup of frost adversely affects the cooling efficiency of the evaporator due to two cooperating factors. First, frost is a thermal insulator. The thicker the frost layer on the evaporator coils, the less efficient the heat transfer between the air and the evaporator. In addition, the buildup of frost restricts the air flow through the evaporator coils. As a result, less air is cooled. Eventually, as frost builds up, the combined effects of reduced air flow and reduced heat transfer require that the evaporator be defrosted to restore cooling efficiency.
One method for defrosting evaporators in prior systems has been to defrost them automatically and periodically under timed control. The time between the defrost cycles is set by an operator based on experience with the system.
Other prior systems have tried to initiate defrost cycles only when the frost buildup is large enough to adversely impact the cooling efficiency of the refrigeration system. In U.S. Pat. No. 4,123,792, a system is described which measures the power consumed by an electric fan motor which draws air over the evaporator. The principle of operation of this system is that frost buildup on the evaporator impedes air flow. As frost builds, the motor works harder to drive the fan, and when a particular set point for power consumption by the fan is reached, the system presumes that defrost is required and a defrost cycle is initiated. Other systems, such as that shown in U.S. Pat. No. 4,400,949, also use information regarding fan motor power consumption but combine that information with information regarding the temperature of the refrigerated space and the temperature of the unit cooler to determine whether defrost is required.
Other frost detection systems such as those shown in U.S. Pat. Nos. 4,045,971 and 4,232,528, employ photoelectric sensors to detect the level of frost buildup on an evaporator coil. The system in U.S. Pat. No. 4,831,833 uses an air velocity sensor in the air flow path to determine whether defrost should be initiated.
Another prior art system senses the differences in air temperature on each side of the evaporator in the refrigerated space as well as the temperature of the refrigerant leaving the evaporator. The data from the sensors is processed to determine if there is a frost buildup requiring the initiation of the defrost cycle.